This regulator was designed for use with a generator with a higher output voltage. This type of generator can be found on some boats and on vehicles for the emergency services. They are really just an adapted version of the standard alternator normally found in cars. The field winding is connected to the 12 V (or 24 V) battery supply, whereas the generator winding is configured for the AC grid voltage (230 V or 115 V). This AC voltage now has to be kept stable via the 12 V field winding. Although it’s perfectly possible to use a switching regulator for this, we deliberately chose to use the old and trusted 723. The generator is a three-phase type, with the field winding rated for 12 VDC. The output voltage of the generator depends on its revs and the current through the field winding.
Since the output voltage is relatively high, it is fed via optocouplers to the 723, which is used in a standard configuration. The output is fed via driver T1 to two 2N3055’s, connected in parallel, which supplies the current to the field winding.
In the prototype, we used TLP620 optocouplers. These are suitable for use with alternating voltages because they have two antiparallel LEDs at the input. The regulation works quite well with these, with the output voltage staying within a small range across a wide range of revs. However, the sensitivity of the two internal LEDs can differ in this type of optocoupler, since it’s not always possible to ensure during the manufacturing process that the distance between each LED and the phototransistor is exactly the same.
For a more precise regulation, it would be better to use two individual optocouplers per phase, with the inputs connected in anti-parallel and the outputs connected in parallel. In order to ensure that there is sufficient isolation between the primary and secondary side, you should make a cutout in the PCB underneath the middle of each optocoupler. Instead of a BD136 for T1, you could also use a TIP32 or something similar. For T2 and T3 it’s better to use a type with a plastic casing, rather than a TO3 case.